1. Field of the Invention
The invention relates to an improve material and a method of producing the composite material. More particularly, the improved composite material is useful in constructing buildings and other applications where concrete is used, but it is lighter and stronger than concrete.
2. Background of the Prior Art
Concrete is frequently used as a construction material. For example, concrete is cast on structural steel to insulate the structural steel from damage due to a fire. This is necessary because steel quickly loses tensile strength once it reaches temperatures of 1000.degree. F.
For many years, concrete of similar material such as gunite, which is a mixture of sand and cement, have been used to encase steel as a fireproofing material. A significant disadvantage of concrete as a fire protection material is that concrete weighs between 140 and 165 pounds per cubic-foot. This weight increases the load that must be carried by the structural members of a building which uses concrete for fireproofing. In addition, concrete is difficult to install because of the large bulk or mass of concrete required to furnish fire protection.
Concrete is not a good insulator and reacts badly to thermal shock created by the extreme temperatures of a fire. This thermal shock may cause cracks throughout the concrete and spalling where large pieces of the concrete surface fall away from the installation.
Organic materials have been used to furnish fire protection for structural steel. For example, name brands of organic materials used for this purpose include Albiclad, Chartek and Thermo-lag. These materials furnish protection through processes known as intumescense, ablation, and sublimation. These organic systems are expensive and typically fail on a high-rise fire test when the temperature of the test sample elevates rapidly, in five minutes, to 2000.degree. F.
Concrete also suffers from the drawback of poor tensile strength when used as a building material. Typical concrete materials have a tensile strength of approximately 300 pounds per square inch (psi) versus a compressive strength of approximately 3000 psi. Consequently, concrete is not generally used alone as a structural material, and instead it must be supplemented by a high tensile strength material such as steel.
The weight disadvantage of ordinary concrete can be reduced by increasing the amount of air entrained within the concrete. However, ASTM C-150 limits the acceptable air content in concrete to a maximum of twenty-two percent.
Known concrete materials have also used perlite as an additive to overcome the disadvantage of concrete concerning its weight. These perlite-added concrete materials are less dense than ordinary concrete with densities ranging from 25 to 85 pounds per cubic-foot.
The perlite used as an additive in known concrete material consists of either commercially used perlite which has particle sizes greater than 30 mesh on the Tyler Standard Screen Scale or perlite fines which have particle sizes ranging from 8 to 200 mesh.
The commercially used perlite and perlite fines are expanded before they are used as an additive in known concrete materials. The expansion process consists of rapidly heating the perlite particles with the result that they pop like popcorn.
The known perlite-added concrete materials tend to be soft and friable, with a hardness on the shore D scale in the range of 45 to 50. They also have a low compressive strength of 500 to 800 psi, and a low tensile strength of approximately 300 psi.
The strength characteristics of known perlite-added concrete materials have been improved by applying pressure to the wet mix of perlite, cement, and water before it dries into the final product (U.S. Pat. No. 3,565,650). The resulting perlite-added concrete material has a compressive strength of approximately 1200 psi and a tensile strength of approximately 700 psi.
Concrete and concrete derived materials are also used in prefabricated structural and decorative components of buildings. For example, concrete is typically used for face panels in prefabricated buildings because concrete can be cast into panels which have a selected texture. Alternatively, concrete can be treated to leave an exposed aggregate surface.
Since concrete is a heavy material, the structure must be designed to carry the heavy load exerted by the panels. This structural load is handled by upgrading the load bearing capacity of footings and the structural skeleton. This increases the cost of the overall structure since more material is used and additional labor is required.
Because concrete contains a large amount of aggregate, panels cast from concrete are grainy and are difficult to cast with a fine, detailed surface. Moreover, the minimum thickness of concrete panels is generally considered to be three to four inches because the flexural strength of concrete is low. Since concrete panels must be relatively thick, the cumulative weight of the panels is a disadvantage.
While concrete and concrete derived materials can be cast into many different shapes and forms, such as roof tiles, the weight of concrete roof tiles requires greater structural capacity than would a lighter material.
Accordingly, a need exists for an improved composite material which is lighter and stronger than concrete, has better fireproofing capabilities, and can easily be formed into different products. The composition should be inexpensive to manufacture and easy to install.